I. Field of the Invention
The present invention relates to a humidity-sensitive element.
II. Description of the Prior Art
Various types of devices are known for measuring the water vapor content in the air, that is, humidity. A psychrometer is a typical example of such devices and is inexpensive. However, the psychrometer requires periodic maintenance such as a supply of water. A device for measuring humidity is known which uses a microwave or a laser beam and which measures the humidity in accordance with the absorption and scattering of such electromagnetic waves by water vapor. However, such devices are large in size and are expensive, and for these reasons are not generally used.
Humidity-sensitive elements which utilize the adsorption of water vapor on the surface of (or inside) a solid material have been recently proposed, and some elements of this type are commercially available. Such an element provides a measurement in accordance with a change in an electric resistance due to a change in the humidity. This type of element has a simple structure and is easy to handle, and provides a measurement of (a change in) humidity in the form of an electric signal. For this reason, application of such an element in various different fields is expected.
The use of humidity-sensitive polymers and porous metallic oxides as a material for such a humidity-sensitive element has been widely discussed.
When a porous metal oxide is used, the powder of a selected metal oxide is sintered at a high temperature. A gold paste or ruthenium oxide paste as an electrode material is baked on the element at 800.degree. C. and is securely adhered thereto. The element has no tendency of electrode separation or the like, and the element provides stable operation.
The electric resistance of a metal oxide sintered body which changes in response to a change in humidity is the surface resistance as described above. A change in the electric resistance of such a material is considered to be attributable to the migration of protons present on the surface of or on internal pore inner surfaces of the sintered body through a medium of water molecules introduced by adsorption. In other words, the surface resistance is influenced by the number and mobility of protons, which factors change in accordance with the number of adsorbed water molecules (as a function of humidity). In most cases, a metal oxide, whether a p- or n-type semiconductor or insulator, has, at normal temperature, a smaller surface resistance or electric resistance with an increase in humidity (with an increase in the number of adsorbed water molecules).
However, the water molecules, which are initially in a physically adsorbed state, are shifted to a chemically adsorbed state as time elapses. Then, the mobility of protons in the humidity-sensitive element is decreased, thereby increasing the surface resistance or electric resistance of the element. In addition, when the surface of or pore inner surfaces of the humidity-sensitive element adsorbs together with water vapor a small amount of oil mist, dust, or gases, the humidity-sensitive resistance range of the element may change or, in some cases, the element may subsequently fail to exhibit humidity-sensitive characteristics (response in an electric resistance to a change in humidity).
In order to solve such problems, a method (heating cleaning method) has been proposed. According to this method, a heater is arranged to surround a humidity-sensitive element. Prior to actual operation of the humidity-sensitive element, the element is sufficiently heated so as to eliminate therefrom chemically adsorbed water molecules, oil mist, dust, or gases, thereby providing a humidity-sensitive element having initial characteristics. Although this cleaning method can provide a high detection precision, it does not allow measurement while the element is being cleaned. In other words, this method prevents continuous measurement. However, a method is plausible wherein the surface of a metal oxide is modified by a suitable surface treatment. Continuous measurement is enabled by thus utilizing reversible physical adsorption/desorption of water.
A method described in Japanese Patent Disclosure (Kokai) No. 57-34301 and the like proposes the carrying of a simple substance of phosphorus or sulfur or an oxide thereof on the surface of a metal oxide so as to stabilize humidity-sensitive characteristics of the element. This method allows continuous measurement over a long period of time when the element is placed in the air. However, when the element is exposed to a high humidity of 90% for a long period of time, the output drift is significant.